Grinding tools, and especially wheels have significant commercial applicability to operations such as cutting, shaping and polishing industrial materials. These wheels generally comprise abrasive grain held together by a bonding material in a disk structure. Usually a central bore through the wheel accepts a power driven shaft that permits the wheel to rotate with the abrasive surface in operative contact against a work piece.
The abrasive material is, of course, an important parameter that determines performance of a grinding tool. The art now recognizes at least two broad categories of industrial grain materials, namely "superabrasives" and "conventional abrasives". The former are ultra hard materials which are able to abrade the hardest, and therefore, the most difficult to cut work pieces. The most well known superabrasives are diamond and cubic boron nitride ("CBN"). Conventional abrasives are abrasives which are not as hard as superabrasives and thus find general purpose utility in a wide variety of normally less demanding grinding applications.
Conventional abrasive grinding wheel construction has developed differently from that of superabrasive wheels. Conventional abrasive wheels are generally characterized by a single region of abrasive grain embedded in a bond. That is, the abrasive region extends from the bore outward to the periphery of the wheel. In contrast, superabrasive wheels usually include a core, often of metal, which extends from the bore outward to a cutting surface. The superabrasive is affixed to the circumference of the cutting surface, either as a single layer bonded to the metal core or as a multi-layer, but shallow depth continuous or segmented rim of grain embedded in a bond. The rim, whether continuous or segmented, is fastened to the metal core. The metal core frequently constitutes the major fraction of the solid volume occupied by the wheel, and thus obviates having to fill the wheel from bore to periphery with superabrasive grain and bond. In effect, the core significantly reduces the cost of a superabrasive tool by placing the abrasive grain only at the cutting surface.
Provided that all operating variables are the same, superabrasives usually outperform conventional abrasives in a given grinding application. That is, such performance parameters as speed of removing the work; service life, i.e., volume of work removed per unit of abrasive removed; amount of force needed to push the tool into the work; and power necessary to cut a given hardness work piece, are usually better for superabrasives than conventional abrasives. Hence, it is theoretically desirable to employ superabrasive tools universally. Unfortunately, the cost of superabrasive is typically multiple orders of magnitude higher than conventional abrasive. Consequently, tools of superabrasive grain normally are selected only for jobs in which the work piece material is difficult for conventional abrasive and for jobs demanding very high performance.
In addition to high cost, superabrasive wheels have certain other undesirable characteristics. Significant among these is that the wheel is difficult to dress by virtue of the intrinsically ultra hard nature of superabrasive. This affects wheel manufacture and use in several ways. For example, in wheel fabrication, the fully assembled tool must be "trued" to precisely shape the cutting surface to design tolerances. In operation, the wheel must be periodically dressed to rejuvenate dulled cutting surfaces. Truing and dressing are normally performed by running the wheel against another precisely shaped abrasive material. These operations are slow and difficult because the hardness of the superabrasive is on par with that of the shaped material. It is also difficult to create superabrasive tools with intricately contoured cutting surfaces because the tools necessary to true and dress such contoured tools are not generally available.
It is very desirable to obtain grinding performance from a conventional abrasive grinding wheel that approaches the performance of a superabrasive wheel in appropriate applications, i.e., for cutting a work piece within the hardness range of conventional abrasive capability. It has been discovered that such "near superabrasive performance" can be achieved by operating certain conventional abrasive grinding wheels in ultra high speed mode. That is, the tangential contact speed of the conventional abrasive segment relative to the work piece should be at least about 125 m/s. The stress of operation at such ultra high speeds will cause many wheels, especially traditional conventional abrasive wheels, to rupture and disintegrate. Thus it is important that the conventional abrasive wheel operated in accordance with the present invention be fabricated in such a manner as to possess minimum core strength and rim strength parameters, described in greater detail, below.
Accordingly, there is now provided by the present invention a method of grinding a hard material comprising:
providing a grinding tool consisting essentially of PA1 moving the abrasive segment at a tangential contact speed of at least about 125 m/sec in contact with the hard material.
a core having a core strength parameter of at least 60 MPa-cm.sup.3 /g; PA2 a cement between the abrasive segment and the core; and
an abrasive segment affixed to the circumference of the core, wherein the abrasive segment comprises conventional abrasive grains embedded in a bond having a rim strength parameter of at least 10 MPa-cm.sup.3 /g; and
There is further provided a method of making a grinding tool having an abrasive segment comprising a conventional abrasive and a vitrified bond, in which the grinding tool is adapted to engage a work piece at a tangential contact speed of at least 125 m/s.